The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. One such electrical measurement technique is utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Any references to the determination or use of resistivity in this application are intended to generically include conductivity as well. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Prior art logging while drilling tools utilized to measure formation resistivity, typically utilize one or more wound toroidal core antennas (also referred to as toroidal transmitters and toroidal receivers) deployed in an insulating media along the exterior of the drill collar. As generally described in the prior art, the wound toroidal core antenna induces an electrical current in the drill collar. The electrical current enters the formation on one side of the toroidal transmitter and returns to the drill collar on the other side of the toroidal transmitter. Measurement of the current enables a formation resistivity to be determined.
For example, Redwine et al., in U.S. Pat. No. 3,408,561, disclose an LWD apparatus in which a toroidal receiver is deployed about a drill collar near the drill bit and a toroidal transmitter is deployed about the drill collar uphole of the toroidal receiver. In use, the voltage induced in the toroidal receiver provides an indication of the resistivity of the formation. Aarps, in U.S. Pat. No. 3,305,771, discloses a similar apparatus, but including a pair of toroidal transmitters and a pair of toroidal receivers.
Clark et al., in U.S. Pat. No. 5,235,285, disclose a technique intended to provide vertically and azimuthally resolved resistivity at multiple depths of investigation. An LWD tool including a tubular drill collar having longitudinally spaced first and second wound toroidal core antennas is utilized. The upper antenna is configured as a transmitter while the lower antenna is configured as a receiver. The tool further includes three longitudinally spaced button electrodes deployed in the drill collar between the wound toroidal core antennas. The button electrodes are intended to provide a return path for electrical current flow from the formation to the drill collar, with the current in the button electrodes being measured to determine a lateral resistivity of the regions of the formation opposing the electrodes. The longitudinal spacing of the button electrodes is intended to provide vertically resolved resistivity at multiple depths of investigation. Clark et al. further disclose rotating the drill collar to obtain azimuthally resolved resistivity.
The above described prior art resistivity tools are similar in that each includes two or more wound toroidal core antennas (one configured as a transmitter and the other configured as a receiver) deployed about a drill collar. These antennas create inductive impedances along the otherwise highly conductive drill collar. It is also known in the art to use such inductance in impede the unwanted flow of electrical current into other sections of the drill string or bottom hole assembly. For example, in one such device, magnetically permeable rings are deployed about an electrically conductive drill collar. The rings are positioned below a resistivity tool having wound toroidal antennas, and thus increase the electrical impedance between the resistivity tool and the adjacent bottom hole assembly. A protective, fiberglass sleeve may be deployed around the magnetically permeable rings to reduce the risk of mechanical damage to the rings. This type of device is sometimes referred to as an inductive choke.
While prior art LWD resistivity tools have been used successfully in comrnercial drilling applications, utilization of a multiple turn toroidal transformer is often problematic. A typical wound toroidal core antenna has a primary winding including many turns of insulated wiring about a toroidal core. Construction and protection of the relatively large toroidal core (e.g., typically having a diameter in the range of 4 to 10 inches) and winding are problematic, especially for use in the demanding downhole environment associated with geophysical drilling. Wound toroidal core antennas utilized in drilling applications are subject to high temperatures (e.g., as high as 200 degrees C.) and pressures (e.g., as high as 15,000 psi) as well as various (often severe) mechanical forces, including, for example, shocks and vibrations up to about 650 G per millisecond. Mechanical abrasion from cuttings in the drilling fluid and direct hits on the antenna (e.g., from drill string collisions with the borehole wall) have been known to damage wound toroidal core antennas. Moreover, it is typically expensive to fabricate and maintain wound toroidal core antennas capable of withstanding the above described downhole environment.
Therefore, there exists a need for an improved apparatus for making azimuthally sensitive resistivity measurements of a subterranean formation. In particular, an apparatus not requiring a wound toroidal core antenna may be potentially advantageous for making such azimuthally sensitive resistivity measurements in LWD applications.